# unicorn/prefer-object-from-entries

> Require `Object.fromEntries()` to turn key-value pairs into an object

Building an object with `reduce` and a spread copies the whole object on every step, so the work grows with the square of the number of items. `Object.fromEntries()` does it in one pass and says what the code is for.

The rule reports a `reduce` that starts from `{}` or `Object.create(null)`. The callback must be an arrow function without a block body that returns `{ ...result, key: value }` or `Object.assign(result, { key: value })`. It also reports `_.fromPairs()` and `lodash.fromPairs()`.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-object-from-entries" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-object-from-entries" = ["error", {
  functions = ["_.fromPairs", "lodash.fromPairs"],
}]
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad {"count": 3} */}
```js
const byId = users.reduce((result, user) => ({ ...result, [user.id]: user }), {});

const ages = users.reduce((result, user) => Object.assign(result, { [user.name]: user.age }), {});

const query = _.fromPairs(pairs);
```

Not reported:

{/* good */}
```js
const byId = Object.fromEntries(users.map(user => [user.id, user]));

const ages = Object.fromEntries(users.map(user => [user.name, user.age]));

const query = Object.fromEntries(pairs);
```

A `reduce` that starts from an object with properties, or that adds more than one property per step, isn't reported:

{/* good */}
```js
const settings = flags.reduce((result, flag) => ({ ...result, [flag]: true }), { debug: false });

const index = users.reduce((result, user) => ({ ...result, [user.id]: user, [user.email]: user }), {});
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `functions` | `["_.fromPairs", "lodash.fromPairs"]` | Functions to report when they're called with one argument. A name can have dots, like `"utils.toObject"`. Your list replaces the default. |

{/* bad {"options": [{"functions": ["utils.toObject"]}]} */}
```js
const query = utils.toObject(pairs);
```

`_.fromPairs` isn't in the list anymore:

{/* good {"options": [{"functions": ["utils.toObject"]}]} */}
```js
const query = _.fromPairs(pairs);
```